1. Field of the Invention
The present invention relates to a molding die assembly for a rubber member, such as a closure to be inserted in a mouth of a container, or a piston to be inserted into a syringe. The present invention also relates to a rubber member produced thereby.
2. Description of the Related Art
The above mentioned rubber member, having a cylindrical shape, is conventionally formed by a compression molding process. Namely, a masticated rubber material is preformed into a flat sheet of rubber as a raw material for molding, and the rubber sheet is molded into a predetermined shape using a molding die assembly including an upper die and a lower die having a plurality of cavities. The shortest (closest) distance between the upper die and the lower die of the die assembly is determined so that an appropriate distance is provided between opposed surfaces of the die surface portions of the upper and lower dies, other than the cavities. Rubber members molded in the cavities are interconnected by a thin rubber sheet extending between the opposed surfaces of the upper and lower dies. The rubber members removed from the die assembly, which are interconnected by the thin rubber sheet, and are cut from the thin rubber sheet, so that individual rubber members can be obtained.
FIG. 8 shows an example of a molding die assembly for a rubber piston 31. A lower die 10 is provided with a plurality of substantially cylindrical cavities 11 corresponding to the shape of the rubber piston 31. An upper die 20 is provided with projections with external threads (male screw portions) 25 which are inserted into the corresponding cavities 11 of the lower die 10, but do not contact the lower die 10. The masticated rubber material which has been preformed in a flat sheet is fed between the lower die 10 and the upper die 20. The die surface portion of the lower die 10 which is opposite to the upper die 20, other than the cavities 11, defines a flat surface 12. The closest distance “d” between the flat surface 12 and the upper die 20 upon molding is determined in accordance with the thickness of the masticated rubber material preformed in the form of a flat sheet. In general, the closest distance “d” is determined to be identical to the thickness of the masticated rubber material preformed in the form of a flat sheet.
FIG. 9 shows a molded product (sheet) 30 produced by the molding die assembly shown in FIG. 8, having a plurality of rubber pistons 31 which are interconnected by a thin rubber sheet 32. The rubber pistons (cylindrical rubber pistons) 31 are cut from the connecting rubber sheet 32 and are separated from one another, as shown in FIG. 10, in which the cut surface 33 is exposed. The rubber piston 31 is provided on the center portion thereof with a threaded hole (female screw portion) 35 formed by the threaded projection 25 of the upper die 20, so that a plunger of a syringe can be screw-engaged into the threaded hole (internal thread) 35.
FIG. 11 shows an example of a known molding die assembly for a rubber closure. The lower die 10 is provided with a plurality of cylindrical cavities 11 corresponding to the shape of the rubber closure. The upper die 20 is provided with a plurality of cavities 21 whose diameter is larger than the diameter of the cavities 11 and which are coaxial to the corresponding cavities 11. The die surface portion of the lower die 10 which is opposite to the upper die 20, other than the cavities 11, defines a flat surface 12. The die surface portion of the upper die 20 which is opposite to the lower die 10, other than the cavities 21, defines a flat surface 22. The closest distance “d” between the flat surfaces 12 and 22 upon molding is determined depending on the thickness of the masticated rubber material preformed in the form of a sheet. The molded product 40 thus obtained includes a plurality of rubber closures (molded members) 41 interconnected by a connecting rubber sheet 42. The individual rubber closures 41 are cut at cut surfaces 43.
The cut surfaces 33 or 43 of the above described individual rubber pistons 31 and rubber closures 41 are exposed. However, the exposed cut surfaces 33 or 43 are adhesive, so that if the exposed surfaces 33 or 43 are brought into contact with each other, the cut surfaces may adhere to each other. Moreover, when a large number of rubber pistons 31 or rubber closures 41 are conveyed along a conveyance line, they may interfere with a part of the conveying apparatus, interrupting the conveyance operation. Furthermore, the cut surface 33 of the rubber piston 31 increases the sliding resistance with respect to the inner barrel of the syringe. In addition to the foregoing, if the cut surface is large, i.e., if the axial length of the cut surface is large, the thickness of the connecting rubber sheet 32 or 42 is also large, thus resulting in an increased amount of wasted rubber material.
Obviously, the size of the cut surface can be, in theory, reduced by decreasing the closest distance “d” between the lower die 10 and the upper die 20. However, since the size of the cut surface is determined depending on the thickness of the masticated rubber material as mentioned above, it is very difficult in practice to make the cut surface smaller. In particular, if the masticated rubber material preformed in the form of a sheet is too thin, mold defects occur. To prevent this, in general, the thickness of the masticated rubber material to be fed between the upper and lower dies is set relatively large. Under these circumstances, a reduction in the size of the cut surfaces is restricted in the conventional molding die assembly.